rsync is typically used to synchronize files and directories between two different systems. For example, if the command rsync local-file user@remote-host:remote-file is run, rsync will use SSH to connect as user to remote-host.[5] Once connected, it will invoke the remote host's rsync and then the two programs will determine what parts of the file need to be transferred over the connection.

rsync can also operate in a daemon mode, serving files in the native rsync protocol (using the "rsync://" syntax).

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Andrew Tridgell and Paul Mackerras wrote the original rsync, which was first announced on 19 June 1996.[1] Tridgell discusses the design, implementation and performance of rsync in chapters 3 through 5 of his Ph.D. thesis in 1999.[9] It is currently maintained by Wayne Davison.[10]

Similar to rcp and scp, rsync requires the specification of a source and of a destination; either of them may be remote, but not both.[11] Because of the flexibility, speed and scriptability of rsync, it has become a standard Linux utility, included in all popular Linux distributions. It has been ported to Windows (via Cygwin, Grsync or SFU[12]) and Mac OS.

...where SRC is the file or directory (or a list of multiple files and directories) to copy from, and DEST represents the file or directory to copy to. (Square brackets indicate optional parameters.)

rsync can synchronize Unix clients to a central Unix server using rsync/ssh and standard Unix accounts.[citation needed] It can be used in desktop environments, for example to efficiently synchronize files with a backup copy on an external hard drive. A scheduling utility such as cron can carry out tasks such as automated encrypted rsync-based mirroring between multiple hosts and a central server.

By default rsync determines which files differ between the sending and receiving systems by checking the modification time and size of each file. As this only requires reading file directory information, it is quick, but it will miss unusual modifications which change neither.

rsync performs a slower but comprehensive check if invoked with --checksum. This forces a full checksum comparison on every file present on both systems. Barring rare checksum collisions this avoids the risk of missing changed files at the cost reading of every file present on both systems.

The rsync utility uses an algorithm invented by Australian computer programmer Andrew Tridgell for efficiently transmitting a structure (such as a file) across a communications link when the receiving computer already has a similar, but not identical, version of the same structure.

The recipient splits its copy of the file into chunks and computes two checksums for each chunk: the MD5hash, and a weaker but easier to compute 'rolling checksum'.[18] It sends these checksums to the sender.

The sender quickly computes the rolling checksum for each chunk in its version of the file; if they differ, it must be sent. If they're the same, the sender uses the more computationally expensive MD5 hash to verify the chunks are the same.

The sender then sends the recipient those parts of its file that did not match, along with information on where to merge these blocks into the recipient's version. This makes the copies identical. There is an unlikely probability that differences between chunks in the sender and recipient are not detected, and thus remain uncorrected. With 128 bits from MD5 plus 32 bits from the rolling checksum, the probability is on the order of 2−(128+32) = 2−160.

If the sender's and recipient's versions of the file have many sections in common, the utility needs to transfer relatively little data to synchronize the files. If typical data compression algorithms are used, files that are similar when uncompressed may be very different when compressed, and thus the entire file will need to be transferred. Some compression programs, such as gzip, provide a special "rsyncable" mode which allows these files to be efficiently rsynced, by ensuring that local changes in the uncompressed file yield only local changes in the compressed file.

Rsync supports other key features that aid significantly in data transfers or backup. They include compression and decompression of data block by block using zlib, and support for protocols such as ssh and stunnel.

The rdiff utility uses the rsync algorithm to generate delta files with the difference from file A to file B (like the utility diff, but in a different delta format). The delta file can then be applied to file A, turning it into file B (similar to the patch utility). rdiff works well with binary files.

rdiff-backup maintains a backup mirror of a file or directory either locally or remotely over the network, on another server. rdiff-backup stores incremental rdiff deltas with the backup, with which it is possible to recreate any backup point.[19]

The librsync library used by rdiff is an independent implementation of the rsync algorithm. It does not use the rsync network protocol and does not share any code with the rsync application.[20] It is used by Dropbox, rdiff-backup, duplicity, and other utilities.[20]

Duplicity is a variation on rdiff-backup that allows for backups without cooperation from the storage server, as with simple storage services like Amazon S3. It works by generating the hashes for each block in advance, encrypting them, and storing them on the server. It then retrieves them when doing an incremental backup. The rest of the data is also stored encrypted for security purposes.

rsyncrypto is a utility to encrypt files in an rsync-friendly fashion. The rsyncrypto algorithm ensures that two almost identical files, when encrypted with rsyncrypto and the same key, will produce almost identical encrypted files. This allows for the low-overhead data transfer achieved by rsync while providing encryption for secure transfer and storage of sensitive data in a remote location.[21]

As of Mac OS X 10.5 and later, there is a special -E or—extended-attributes switch which allows retaining much of the HFS file metadata when syncing between two machines supporting this feature. This is achieved by transmitting the Resource Fork along with the Data Fork.[22]

Acrosync is an independent, cross-platform rsync implementation that is not based on the rsync source code. Native Windows, Mac OS X, and iOS clients are currently available.[23]

zsync is a rsync like tool optimized for many downloads per file version. zsync is used by Linux distributions such as Ubuntu[24] for distributing fast changing beta ISO image files. zsync uses the HTTP protocol and .zsync files with pre-calculated rolling hash to minimize server load yet permit diff transfer for network optimization.